1. Field of the Invention
The invention relates in general to a structure of a flash memory, and more particularly to a flash memory structure with less body effect.
2. Description of the Related Art
Nonvolatile memory is widely applied in a variety of electrical devices such as storing structure data, program data and other multiple writing and reading data. Within the programmable nonvolatile memories, electrically erasable and programmable nonvolatile memories (EEPROMs) are now of great interest since they are widely applied in personal computers and electrical equipment. Conventional EEPROMs include floating gate transistor structure, which has the advantages of writing, erasable, and storing data but also has the disadvantages of low speed. However, the newly developed flash memory structure EEPROM has higher speed of writing and reading.
Typically, n-channel EEPROM and flash EEPROM devices adopt a selector transistor in an array of memory cells or a sector of memory cells. In the latter case, the selector transistor is often called a sector select gate. Enhancement mode transistor is typically used as a selector transistor. The function of the selector transistor is to isolate memory cells and common bit lines connected to a large amount of memory cells. Referring to FIG. 1, it shows the structure of a conventional NAND memory cell. During the operation of reading, the select gate 100a of the selected memory cell 100 is turned on so that the data stored at the floating gate of the memory cell can be tested by a sense amplifier. The select gates 104a, 106a of the other memory cells 104, 106 are turned off so that the non-selected memory cells 104, 106 will not disturb the operation of reading. Conclusively, the enhancement mode n-channel transistor is used as a select gate.
During the operation of programming, the function of the selector transistor is also to isolate the non-selected memory cells and the bit line. Referring to FIG. 2, generally, a high voltage of about 12-20 V is provided to the bit line 200. For a selected memory cell 202, since the select gate 202a is turned on, the high voltage on the bit line can be transferred to the drain region of a selected floating gate transistor. The high voltage induces Fowler-Nordeim tunneling so that the electrons stored at the floating gate go into the tunneling window to complete the operation of programming. For the non-selected memory cells 204, 206, since the select gates 204a, 206a are turned off as the bit line 200 is kept in the state of high voltage, the charges stored at the floating gate can be prevented from the influence of programming.
During the above-mentioned operation of programming, body effect often occurs. Referring to FIG. 3, it illustrates the body effect of a n-channel MOSFET. As the voltage provided to the drain electrode P is 10 V and the voltage provided to the gate electrode R is 5 V, the voltage of the source electrode Q approaches (5 V-V.sub.T), wherein V.sub.T represents threshold voltage. The threshold obtained by the foregoing method is much higher than the one of an equal MOSFET device with its source electrode grounded. This effect is therefore called body effect. For example, as the source electrode of the MOSFET device is grounded, the threshold voltage of it is about 0.7 V. On the other hand, the device as shown in FIG. 3 has a threshold voltage of nearly 1.2 V and a source voltage of 3.8 V due to source bias. It is therefore apparent that as the source electrode is in the state of reverse bias there is a tendency of increasing threshold voltage. As a result, the voltage provided to the selector transistor gate R is necessarily higher than the voltage provided to the drain electrode, which is about 2-5 V, to overcome the body effect of the enhancement mode select gate. However, higher voltage increases the energy consumption and cost.